Antifuses, which are widely used in integrated circuits, are programmable structures having an extremely high resistance (e.g., greater than 100 M.OMEGA.) prior to programming and a low resistance (e.g., 200-400 .OMEGA.) after programming. In general terms, a programmed antifuse establishes an electrical connection at a location where an open circuit previously existed. For example, antifuses placed at the locations in a logic array where horizontal and vertical wires intersect may be selectively programmed to make electrical connections only at the desired locations, thereby programming the array logic component to perform a specific function.
Before being programmed, a typical antifuse structure may consist of a dielectric layer between two conducting electrodes. For example, a thin silicon nitride dielectric layer may be sandwiched between two conductors such as an aluminum layer and a diffusion layer.
Such antifuses may be "blown" or programmed by applying a voltage across the two electrodes sufficient to rupture the dielectric layer and create a conductive filament through the dielectric layer to connect the two electrodes. The resistance of the antifuse after programming depends upon various factors. These factors include (a) the particular dielectric material used, such as silicon nitride, silicon dioxide or other thin films, and (b) the voltage and current used to program the antifuse.
If the antifuse is satisfactorily programmed, the antifuse is permanently changed from a high-resistance or "unprogrammed" value (e.g., 100 M.OMEGA.) to a low-resistance or "programmed" value (e.g., 200-400 .OMEGA.). Unfortunately, the antifuse may not be satisfactorily programmed, but may instead be defectively programmed. For example, normal processing variations may result in the antifuse having a value of 3-5 K.OMEGA. rather than the desired 200-400 .OMEGA.. If processing control is not properly maintained, the antifuse may have an even higher resistance, for example 10-12 K.OMEGA..
Antifuses are used in various applications, including logic control circuits which are programmed by blowing the antifuses. Sense amplifiers are used to "read" such control circuits by sensing whether the antifuses are in their programmed or unprogrammed state. Defectively programmed antifuses can cause such control circuits (or other types of circuits using the antifuses) to malfunction. For example, a conventional sense amplifier circuit may erroneously read an antifuse which has a resistance of, e.g., 400-800 .OMEGA. or higher as being unprogrammed. Under normal processing conditions, only a small percentage of antifuses (e.g., one a thousand) will be defectively programmed. However, if a single antifuse is defective, the entire "die" or chip containing the defective antifuse may have to be discarded. Since the antifuses normally occupy only a very small portion (e.g., 0.1 percent) of the die, this significantly reduces the yield.